The present invention relates, in general, to semiconductor switches, and more particularly, to semiconductor switches with integral on-state voltage detection capability.
Power transistors, including MOSFETs, bipolar transistors, and insulated gate bipolar transistors (IGBTs) are well known devices for switching high power loads. High power switches have two current carrying electrodes and a control electrode and are characteristically able to control high current, high voltage, or both. Because power switches are exposed to high currents and voltages, there exists a constant danger of overheating resulting in permanent damage to the semiconductor switch.
High power switches must support high voltage when the device is off and support high current when the device is on. When a power switch is off, little current flows between the current carrying electrodes and so large voltages across the current carrying electrodes dissipate little power in the device. When a power switch is on, voltage across the current carrying electrodes is normally quite low, thus large currents flow through the current carrying electrodes with high but manageable power dissipation.
Designers of power control circuits, power control modules, and power semiconductor devices are concerned with protection of power switches. Control circuits using many discrete components or integrated circuits are often used to detect dangerous conditions and turn the power switch off to limit power dissipation. These control circuits monitor device temperature with a temperature sensor located near the power switch, or monitor current through the device. One problem with temperature sensing circuits is they react slowly to a dangerous condition because thermal signals are transmitted much slower than electrical signals. Temperature sensing circuits also require elaborate and expensive packaging and assembly to place a temperature sensor sufficiently close to the power switch.
Current sensing circuits can limit power dissipation effectively, but at a great sacrifice in power switching capability. High currents are usually dangerous only when an excessive on-state voltage simultaneously appears across the current carrying electrodes. Current sensing and limiting circuits do not prevent the semiconductor switch from operating in an excessive on-state voltage condition, but instead prevent high currents which could cause damage only if the device did enter the high on-state voltage region. Current sensing circuits are thus more appropriate for preventing damage to external components which are driven by the power switch than for protecting the power switch itself.
Accordingly, it is desirable to have a high power semiconductor switch which detects an excessive on-state voltage condition, while not limiting the switch's current carrying or voltage switching capability.